[CFG] Add iterator_ranges to CFG and CFGBlock.

Summary: (Needed for D23353.)

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D23842

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@281200 91177308-0d34-0410-b5e6-96231b3b80d8